Polyhydroxyalkanoates (PHAs) are natural polyesters produced by a large variety of microorganisms such as bacteria and algae. They are biodegradable thermoplastics obtained from renewable sources that can be processed with conventional equipment, which makes them very attractive for the plastic industry. The potential worldwide market for biodegradable polymers is enormous due to the extreme variety of applications. For example, degradable polymers can be used as films, sheets, fibers, foams, molded articles and many other products.
PHAs produced by microorganisms are intracellular granules accumulated as energy storage resulting of adverse growth conditions, i.e., nutrient limitation. The biopolymer accumulation in bacteria increases when a deficiency in nitrogen occurs. This deficiency is generally expressed by an increase of the ratio C/N, where C is the source of carbon and N the source of nitrogen actually in the culture medium. Therefore, the feeding strategy becomes a critical step that will have a direct impact on the productivity of the biopolymer. The food source is also an important factor that will decide the nature of the produced biopolymer. In fact, different homo- or copolymers can be obtained by varying the food source provided to the microorganism during the fermentation. The most well-known representatives of the PHA family are poly(3-hydroxybutyrate) (PHB) as well as its copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV).
PHAs are biopolymers that are characterized by other numerous interesting properties. Among them, they are biocompatible and bioresorbable, which makes PHAs potent candidates for food, cosmetic and biomedical applications. An increasing number of publications and patents over the last years provide the best illustration. Yalpani reported the use of poly(hydroxy alkanoates) as fat substitute for food in the U.S. Pat. No. 5,229,158. Marchessault et al. described the use of PHA for the entrapment or microencapsulation of hydrophilic or lipophilic drugs in the U.S. Pat. No. 6,146,665. In this case, PHA is synthesized in vitro by polymerization of a hydroxyalkanoate coenzyme A monomer. Other controlled applications were published on PHA from bacterial sources.
The potential of PHA in drug delivery systems is now known in the art. PHAs are also used as implants in orthopedic surgery because of their biodegradability and bioresorption. For this particular use, PHAs are often reinforced with hydroxyapatite (Biomaterials, 1991, 12:841-847; Biomaterials, 1992, 13:491-496; Polymer Testing, 2000, 19:485-492). Numerous other implant applications were developed such as heart valves, vascular grafts and tissue engineering. Cosmetic composition containing hydroxy alkanoate derivatives was reported by Browser et al. in the International Patent Pub No. WO 95/05153. In this patent application, oligomers (1 to 5 monomer domains) of 2-hydroxyalkanoate derivatives are incorporated in the composition.
The solubility of these biopolymers is very low. They are totally insoluble in water and in most common organic solvents, which appear to be poor-solvents, with the exception of some halogenated solvents such as chloroform, dichloromethane and 1,2-dichloroethane. Traditionally, PHB is extracted by adding a PHA non-solvent to an halogenated solution containing the biopolymer (U.S. Pat. No. 4,562,245), which is not cost efficient as far as a large scale production is concerned. Therefore, the major concern about the extraction and purification of the biopolymer from the microorganism was the production cost. As a result, a lot of efforts were put forth to resolve this problem and many patents were issued. For example, method using PHA-poor solvent at high temperature (International Patent Pub. No. WO 98/46783), using non-halogenated solvents (International Patent Pub. No. WO 98/46782) and using marginal non-solvents (International Patent Pub. No. WO 97/07229). An aspect of the use of organic solvents at high temperature was the discovery of PHA gels once the solutions were allowed to cool at room temperature. Other examples of the formation of physical gel were found in the literature, Fabri et al. studied dilute solution of PHB in N,N-dimethylformamide and N-methyl-2-pyrrolidone (Thermochimica Acta, 1998, 321:3-16) whereas Turchetto and Cesbro used dimethylformamide (Thermochemica Acta, 1995, 269/270:307-317). The lower degree of solubility of polymers like PHAs in organic solvents was exploited by Dunn and English for drug release applications (International Patent Pub. No. WO 01/35929). These authors used a floating component containing the polymer and a bioactive agent that is administered to human by syringe and needle. Once introduced in the body, the solvent is dispersed and the polymer which is non soluble in water forms a solid matrix where the bioactive agent is trapped and further release.
One aspect of the purification and extraction process is the use of a dispersing agent of PHA in water by the addition of a surfactant (Patent Pub. No. WO 97/21762), but it does not lead to the formation of a gel neither of a cream.
U.S. Pat. No. 5,229,158 describes the use of PHA in a latex solution, with particle sizes that can get from 0.1 to 10 microns, which is similar to our statements. However, the main aggregating agents are totally different, for example pectin, lecithin and xanthan gum. No indication is given regarding the physical aspect of the final product neither its stability in time. PHA is used to substitute fat entities because it has a fat-like texture.
Moreover, the above-described applications and inventions have a limited range of concentration of PHA when organic solvents are used. In fact, it is impossible to obtain over a 5%. PHA solution (weight/volume) in organic solvents.
The stabilization of PHA dispersion in water as been reported in International Patent Pub. No. WO 97/21762. This application describes the use of amphiphilic chemical entities that would improve the solubility of the PHA in water and simplify the process of dispersion in order to purify the biopolymer during the extraction/purification processes. Dispersants used are for example dioctyl sulphosuccinate, sodium dodecylsulphonate, sodium dodecylbenzenesulphonate, sodium lauryl sarcosinate or sodium dodecyldiphenyl oxide disulphonate.
Different biodegradable copolymers have been described until now, including aliphatic polyester, polyorthoester, polyanhydride, poly alpha-amino acid, polyphosphagen, and polyalkylcyanoacrylate. Among aliphatic polyesters, polylactide (PLA), polyglycolide (PGA) and polylactideglycolide (PLGA) were approved as copolymers nontoxic to humans by the FDA. These copolymers were employed as drug delivery devices to carry the drugs or biomedical devices.
Based on the above-listed patents and publications which are quite representative of the state of the art relating to biopolymers, there is still considerable amount of work to do in order to improve the process of producing gels and creams because of the lack for methods to obtain a gel and/or a cream using biopolymers, particularly PHAs, that would be suitable principally for cosmetic and pharmaceutical applications. Such a process would rather use biocompatible and bioresorbing species.
It would be very much desirable to be provided with a new method for producing new biocompatible gels and creams composed with biopolymers.